1. Field of the Invention
The present invention relates to control signal terminating servers and protocol terminating methods in mobile communication networks and, particularly, to a control signal terminating server, a protocol terminating method, and a mobile communication network in a soft handover system without a redundant path.
2. Related Background Art
There is the UMTS (Universal Mobile Telecommunication System) as a conventional mobile communication system standardized by 3GPP (3rd Generation Partnership Project: the standardization project for third-generation (3G) mobile communication systems). The UMTS adopts W-CDMA (Wideband-Code Division Multiple Access) as a radio access technology.
A technical specification group in 3GPP is elaborating UMTS RAN (Radio Access Network).
In the RAN, various radio-related controls, such as soft handover (diversity handover) control, transmit power control, and paging control, are carried out. For implementing these various controls, a variety of protocols are defined in the RAN. A protocol configuration in the RAN is composed of layer 1 (physical layer: L1), layer 2 (data link layer: L2), and layer 3 (network layer: L3) Layer 1 has the handover function, the error correction and detection function, the spectrum spreading modulation/spectrum despreading demodulation function, and the transmit power control function, and in response to a request for transmission of a signal from layer 2, layer 1 supplies the signal to layer 2 through use of a transport channel according to use thereof.
Typical protocols and their controls in the RAN are outlined as follows.
.MAC (Medium Access Control)
MAC is a sublayer of layer 2 and performs multiplexing/demultiplexing of radio layer 2 frames (RLC (Radio Link Control)-PDUs (Protocol Data Units)) onto a transport channel, execution of ciphering/deciphering, measurement of traffic volume and quality, and so on (cf. Non-patent Document 2).
RLC (Radio Link Control)
RLC is a sublayer of a layer 2, and performs segmentation/concatenation/reassembly of layer 3 data, sequence control, retransmission control (ARQ), execution of ciphering/deciphering, and so on (cf. Non-patent Document 3).
.PDCP (Packet Data Convergence Protocol)
PDCP is a protocol of a sublayer of layer 2, and performs appropriate data conversion such as header compression of IP packet, prior to radio transmission (cf. Non-patent Document 4).
.RRC (Radio Resource Control)
RRC is a protocol of layer 3, and performs control of layer 2 protocols, establishment/release of an RRC connection, notification of broadcast information, paging, radio resource control, power control, control of ciphering, and so on (cf. Non-patent Document 5).
.FP (Frame Protocol)
FP performs transfer control of data of wired part, control of channel synchronization and node synchronization for control of downlink data arrival synchronization necessary for soft handover, and so on (cf. Non-patent Document 1).
.RNSAP (RNS Application Part)
RNSAP performs transmission/reception of control signals between SRNC (Serving-RNC) and DRNC (Drift-RNC) (cf. Non-patent Document 6). SRNC is an RNC in a state in which the RNC (Radio Network Controller) is in an RRC connection with a UE (user equipment: mobile unit).
SRNC responds in UTRAN (Universal Terrestrial Radio Access Network), and serves as a connection point to a core network.
DRNC is an RNC to be newly wirelessly connected with a UE when the UE in a connected state is handed over to a cell associated with a different RNS (Radio Network Subsystem). DRNC functions as a switch between SRNC and UE and performs routing of information.
UTRAN is a part consisting of at least one RNC and at least one Node B (radio base station) between Iu interface and Uu interface, in a UMTS network.
The Iu interface is an interface to link the RNC with either a 3GMSC (3G Mobile Switching Center) or a 3GSGSN (3G Serving GPRS Support Node), and the Uu interface is a radio interface between the UTRAN and the UE utilizing CDMA. Node B has a function of providing a physical radio link between the UE and the network.
.NBAP (Node B Application Part)
NBAP performs transmission/reception of control signals between the RNC and the Node B (cf. Non-patent Document 7).
In the UMTS, transactions of user data and transactions of control information are carried out separately from each other.
This user data is called U-Plane (user information transport plane: protocol for control of user data) data (user data). Protocols used in the transactions of U-Plane data are FP, MAC, RLC, and PDCP from the lower level.
The control information is called C-Plane (call control signal plane: protocol for transmission/reception of control information) data (control signal). Protocols used in the transactions of C-Plane data are FP, MAC, RLC, RRC, and, RNSAP and NBAP from the lower level.
(1) U-Plane
FIG. 1 is a block diagram for explaining the U-Plane protocols.
An SRNC 2 is connected to a core network 1, Nodes B 4, 5 are connected to the SRNC 2, and Nodes B 6, 7 to a DRNC 3. The RNCs are, specifically, access routers, and the Nodes B are, specifically, radio base stations.
The SRNC 2, which is an RNC at the time of a start of a communication, and the Node B 5 are connected by FP, and the SRNC 2 and a UE 8 are connected by MAC, RLC, and PDCP. In a case where the UE 8 makes a handover between Nodes B 4, 5 located under the SRNC 2 (Intra-RNC handover), the U-Plane data is directly multicast from the SRNC 2 to each Node B to which the UE 8 is connected.
On the other hand, in a case where the UE 8 makes a handover between Nodes B 5, 6 located under different RNCs 2, 3 (Inter-RNC handover), the U-Plane data is transmitted via SRNC 2 and further via DRNC 3 being a drift RNC, to the destination Node B 6.
In the diversity handover control using the subscriber line extension system in the UMTS, in the case of the Intra-RNC handover, data is multicast from the SRNC 2 to each of Nodes B connected in star topology. For this reason, there arises no problem in the sense of data transmission using an optimal route (shortest route). However, in the case of the Inter-RNC handover, there are a case where the U-Plane data is directly transmitted to Node B 5 under SRNC 2 and a case where the U-Plane data is transmitted from SRNC 2 via DRNC 3 to Node B 6 under DRNC 3.
In the transmission/reception of the U-Plane data, as described above, FP is used between SRNC 2 and Node B 5, between SRNC 2 and DRNC 3, and between DRNC 3 and Node B 6. MAC, RLC, and PDCP are used between SRNC 2 and UE 8.
(2) C-Plane
FIG. 1 is a block diagram for explaining the C-Plane protocols.
The connection relationship of the network is the same as in the U-Plane case. FP is used between SRNC 2 and Node B 5, between SRNC 2 and DRNC 3, and between DRNC 3 and Node B 6. MAC, RLC, and RRC are used between SRNC 2 and UE 8, and RNSAP is used between SRNC 2 and DPNC 3. Furthermore, NBAP is used between SRNC 2 and Node B 5 and between DRNC 3 and Node B 6. The terminal endpoints of the respective protocols on the UMTS architecture are as described above.
As described above, MAC and RLC are common controls used in the C-Plane and in the U-Plane. SRNC 2 is a node serving as a branch/aggregation point of data on the occasion of a soft handover, and it is the RNC having established an RRC connection when the UE 8 started the communication. According to the specification, there exists only one SRNC for one communication and the location thereof is fixed during the soft handover.
[Non-patent Document 1] 3GPP TS 25.427 “UTRAN Iub/Iur interface user plane protocol for DCH data streams”
[Non-patent Document 2] 3GPP TS 25.321 “Medium Access Control (MAC) Protocol Specification”
[Non-patent Document 3] 3GPP TS 25.322 “Radio Link Control (RLC) Protocol Specification”
[Non-patent Document 4] 3GPP TS 25.323 “Packet Data Convergence Protocol (PDCP) protocol”
[Non-patent Document 5] 3GPP TS 25.331 “Radio Resource Control (RRC) Protocol Specification”
[Non-patent Document 6] 3GPP TS 25.423 “UTRAN Iur interface RNSAP signaling”
[Non-patent Document 7] 3GPP TS 25.433 “UTRAN Iub interface NBAP signaling”